the route has to be 

```xml
<routes xmlns="http://symfony.com/schema/routing"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/routing
        http://symfony.com/schema/routing/routing-1.0.xsd">

    <route id="liip_imagine_filter" path="/media/cache/_runtime/{hash}/{filter}/{path}" methods="GET">
        <default key="_controller">%liip_imagine.controller_action%</default>
        <requirement key="filter">[A-z0-9_\-]*</requirement>
        <requirement key="path">.+</requirement>
    </route>

    <route id="liip_imagine_filter" path="/media/cache/{filter}/{path}" methods="GET">
        <default key="_controller">%liip_imagine.controller_action%</default>
        <requirement key="filter">[A-z0-9_\-]*</requirement>
        <requirement key="path">.+</requirement>
    </route>
</routes>
```

And cache manager has to throw exception if one pass filter with the name `_runtime`. Manager has to tell that you cannot use a filter with such name.